DESCRIPTION: Contrast in magnetic resonance images is determined predominantly by differences in nuclear spin relaxation rates. The administration of contrast agents that control proton spin relaxation rates provides control of the information available from MRI. The work proposed addresses fundamentals of magnetic relaxation in paramagnetic systems that are potential contrast agents, with the aim of understanding what intramolecular and dynamical variables control relaxation efficiency. A major issue is to understand clearly what controls electron spin relaxation rates in soluble metal complexes and in more rigid solid systems, since the electron relaxation times usually become the correlation times for the electron-nuclear coupling and limit the paramagnetically induced proton relaxation rate. Dr. Bryant proposes extending work in iron(III) systems with the hope that efficiency comparable to lanthanides can be achieved. Work is proposed in particulate systems, with the goal of understanding relaxation in systems when water may sample a geometrically constrained space that is doped with relaxation centers. The effects of these porous, magnetically active materials depend only weakly on the magnetic field strength, and are potentially useful for gastrointestinal tract agents or other externally accessible spaces such as the bladder. The theoretical understanding is an important foundation for proton spin relaxation in microporous materials like bone. Targeting of contrast agents is the next step beyond present use of non-specific agents. A significant effort is proposed to understand the changes in relaxation efficiency when an agent binds to macromolecular targets that may not rotate freely. Finally, Dr. Bryant proposes to explore quantitatively the relationship between magnetization transfer contrast and paramagnetic contrast agents which, when used in concert, may provide significant dynamic range gains.